Partial Discharge (PD) represents tiny electrical discharges occurring within or on the surface of insulation materials in high-voltage equipment, such as transformers, cables, and switchgear. This phenomenon is a localized dielectric breakdown that does not completely bridge the space between two conductors, hence the term “partial.” PD is a progressive defect highly relevant to equipment longevity because its sustained presence gradually destroys the insulating material. These subtle, repetitive discharges can go undetected for long periods while silently undermining the health of a power system.
What Partial Discharge Is
Partial discharge is a localized failure of an insulating material when the electric field strength in a small region exceeds the material’s dielectric strength. This breakdown occurs only in a small portion of the insulation system, such as a microscopic void or crack, and not across the entire distance between the conductor and the grounded electrode. For example, a small air bubble trapped inside a solid insulator acts as a weak spot. Since air has a much lower dielectric constant than the surrounding solid, the electric field concentrates across the gas-filled void, subjecting it to disproportionately higher electrical stress.
When the voltage across this void reaches a certain level, the gas inside ionizes, causing a minute electrical spark. This discharge lasts for only a fraction of a microsecond, dissipating energy as light, heat, sound, and high-frequency electrical pulses. Because the discharge does not short-circuit the conductors, the voltage across the void quickly drops, extinguishing the spark. This process repeats hundreds or thousands of times per second, slowly eroding the material surrounding the defect.
Factors That Cause PD Generation
The initiation of partial discharge is linked to a physical defect or impurity that causes a non-uniform distribution of the electric field. The most common cause is the presence of internal voids or cavities, which are gas pockets formed within solid insulation during manufacturing. These voids, such as air bubbles in epoxy, become high-stress points where the local electric field exceeds the gas’s breakdown strength, triggering an internal discharge. Poor workmanship, such as insufficient impregnation or improper mixing of compounds, often leads to these microscopic flaws.
Partial discharge can also occur along the surface of an insulator, known as surface discharge or tracking. This is initiated by environmental factors like contamination, moisture ingress, or humidity, which create a thin, conductive film across the insulation surface. A third type, corona discharge, occurs in the gas surrounding a conductor, usually emanating from sharp edges where the electric field is highly concentrated. While corona discharge in open air is generally less damaging to solid insulation, it produces corrosive byproducts like ozone that can chemically attack nearby materials.
Consequences for Electrical Systems
Sustained partial discharge activity causes progressive, irreversible damage to the insulation system. The energy dissipated by the repetitive micro-sparks acts as a continuous bombardment of the insulation material by energetic electrons and ions. This mechanical and electrical erosion gradually enlarges the original defect, creating carbonized paths with significantly reduced insulating properties. Localized heating from energy dissipation also accelerates the thermal degradation of the material, further weakening its structure.
As damage accumulates, the insulation forms a characteristic branching pattern of conductive channels known as “electrical treeing.” This structure grows through the dielectric, progressively reducing the insulation wall until it can no longer withstand the operating voltage. The result is a catastrophic insulation failure, manifesting as an arc flash, leading to equipment breakdown and unplanned power outages. PD-induced failure incurs high economic costs, including replacing expensive high-voltage assets like power transformers and financial losses from extended downtime.
Methods for Detecting Partial Discharge
Since partial discharge is an invisible, internal process, engineers use sophisticated monitoring techniques to detect the physical phenomena accompanying the event.
Electrical Detection
One sensitive method is electrical detection, which measures the high-frequency current pulses generated by the discharge. Sensors like High-Frequency Current Transformers (HFCTs) or Transient Earth Voltage (TEV) sensors are used. TEV sensors detect electromagnetic spikes propagating along the metallic enclosure of switchgear, providing a non-invasive check for internal PD activity.
Acoustic Emission
PD events can also be located using acoustic emission techniques, which capitalize on the minute sound waves produced by the rapid expansion of gas near the discharge site. Ultrasonic sensors pinpoint the precise physical location of the defect, making them particularly useful for finding surface and corona discharges in air-insulated equipment.
Chemical Analysis
A third diagnostic approach is the chemical analysis of insulating fluids, such as dissolved gas analysis (DGA) in transformer oil. PD activity generates specific chemical byproducts, including hydrogen, methane, and acetylene. Monitoring the concentration of these gases provides a clear indication of the severity of the internal electrical activity.
Strategies for PD Prevention
Preventing partial discharge begins with stringent quality control during manufacturing to eliminate internal defects. This involves ensuring a clean environment, using high-quality insulating materials, and employing robust processing techniques, such as meticulous vacuum impregnation to remove air bubbles. Manufacturers must focus on preventing voids and ensuring the uniform curing of solid dielectrics, as these defects are the primary initiators of internal PD.
Design practices are also crucial, requiring the careful shaping of electrodes to minimize electric field concentration and prevent corona discharge inception. Smoothing sharp edges and applying stress-grading materials at critical interfaces, like cable terminations, helps ensure the electric field is distributed evenly across the insulation. Maintenance efforts focus on environmental control, such as sealing equipment against moisture ingress and regularly cleaning external surfaces to prevent surface contamination and tracking.